Use this free compound interest calculator to see how a starting balance and regular monthly deposits can grow over time. Change any number, and the result updates instantly. Nothing you type is saved or sent anywhere.
How this compound interest calculator works
The calculator assumes your return is added to your balance every month, and that each monthly contribution is made at the end of the month. That is how most savings, brokerage and retirement accounts behave in practice. The formula is: balance = starting balance × (1 + r/12)12n + monthly contribution × [((1 + r/12)12n − 1) ÷ (r/12)], where r is the annual return and n is the number of years.
The “today’s dollars” figure divides the final balance by (1 + inflation)years. It answers a more useful question than the raw total: what will that money actually buy? A balance of $313,109 in 30 years sounds large, but at 2.5% inflation it has the buying power of about $149,000 today.
Example: $1,000 plus $250 a month at 7%
With the default numbers above, you contribute $91,000 over 30 years ($1,000 up front plus $250 × 360 months). The calculator shows a final balance of about $313,109, which means roughly $222,000 of the total, around 71%, comes from growth rather than your own deposits.
| Years | Starting from $0, $250/month | Starting from $1,000, $250/month |
|---|---|---|
| 10 | $43,271 | $45,281 |
| 20 | $130,232 | $134,270 |
| 30 | $304,993 | $313,109 |
| 40 | $656,203 | $672,515 |
Notice how the last decade adds more than the first three combined. That is the core lesson of compounding: time matters more than the size of any single deposit. Our guide to compound interest at 25 walks through the same idea with real-world examples.
Choosing a realistic return
No one knows future returns. The 7% default is a common planning figure for a diversified stock portfolio, based on long historical averages, but real years vary widely and some are negative. A savings account may pay far less. Try 4%, 7% and 10% to see a range, and treat the middle number as an illustration rather than a promise.
Fees matter as well. A fund that charges 1% a year takes a meaningful share of long-term growth compared with a broad index fund charging a few hundredths of a percent. Learn how to compare options in index funds vs ETFs.
Ways to get more from compounding
Start as early as you can, even with a small amount. Automate your contributions so they happen without effort. Increase the monthly amount whenever your income rises. Use tax-advantaged accounts first: the 2026 limit is $24,500 for a 401(k) and $7,500 for an IRA. If you are under 30, a Roth IRA can make that growth tax-free. To set a savings target, try the retirement savings calculator or read how much to save for retirement.
Compound interest calculator FAQ
What is the difference between simple and compound interest?
Simple interest is paid only on your original amount. Compound interest is paid on your original amount plus interest already earned, so growth speeds up over time.
Does this calculator include taxes?
No. Taxes depend on the type of account. Growth in a Roth IRA can be tax-free in retirement, while traditional accounts are taxed when you withdraw. Check the IRS retirement plans page for current rules.
Is the result guaranteed?
No. It is a projection based on a constant return you choose. Actual investment returns rise and fall, and the value of investments can drop.
This calculator is for education only and is not personalized financial advice.